This invention relates to a device for influencing the visual information presented to at least one eye of at least one observer, whereby a shutter apparatus is placed between the observer's eye and the source of the visual information. Each shutter apparatus comprises one or more liquid crystal cells, each of which comprises a layer of liquid crystal material surrounded by two parallel plates of glass, the surfaces of which are coated with a transparent conductive film. By introducing an electrical voltage between each pair of plates the light transmission characteristics of each cell can be regulated.
Such an optical device is known from U.S. Pat. No. 4,021,846. That device has as its objective that an observer perceive in three dimensions images generated by a visual display device, by means of synchronizing the operation of the shutter apparatus with alternately presented display images, such that each of the observer's eyes receives information intended for that eye only. The liquid crystal cells in that apparatus are constructed by sandwiching a layer of nematic liquid crystal material between two parallel plates of glass. The surfaces of the plates which are in contact with the liquid crystal material are coated with a transparent conductive film, thus allowing the plates to serve as electrodes, across which an electric field can be applied. The liquid crystal cells used in that invention have the property that the plane of polarization of incident polarized light can be continuously rotated from 0.degree. to 90.degree., depending on the magnitude of the voltage imposed across the electrodes. The liquid crystal cells are placed between crossed polarizers in order to cause the transmission of light through the cell to vary between a minimum and a maximum value, in conjunction with the imposed steering voltage. When suitable images intended for alternate presentation to an observer's eyes are presented on a suitable display device, an appropriate steering signal can be sent to the liquid crystal apparatuses to ensure that each of the alternating displayed images is presented only to the appropriate eye of the observer, in order thus to allow the observer to perceive depth in the resulting composite image. Such images can be, for example, images whose composition is determined by display elements generated by a measuring or computing device, or pictorial images which are produced on a display device such as a television receiver. In all cases, in order to achieve a continuous three dimensional representation, the alternating picture elements must each be presented from a geometrical side-by-side perspective appropriate to the perspective intended to be viewed by the corresponding eye of the observer.
One disadvantage of such a stereoscopic viewing device is that not all wavelengths of incident light are equally affected, resulting in an unsatisfactory reproduction of colour images. In addition, the polarizing filters absorb an appreciable amount of the incident light, which reduces the perceived intensity of the entire visual environment, including the presented display image. Yet another disadvantage is that the switching times between the light and dark states for this device are relatively large, thus necessitating the simultaneous blanking of the images intended for both eyes for an appreciable amount of time during part of the presentation cycle in order to prevent either eye from receiving any information intended for the opposite eye. Consequently, for the high rates of presentation of display images necessary for the perception of continuously fused images, the proportion of time during which both eyes receive no information is relatively large. This can result in a disturbing flickering effect, which is very fatiguing for the eyes of the observer. Another consequence is a reduction in the upper limit on the speed of continuous perceived motion of the three dimensional images which one is able to display.
There do, however, exist liquid crystal cells which may be switched more rapidly and which are also able to influence the transmission of incident light without necessitating the use of polarizing filters with their accompanying disadvantages. Such liquid crystal cells are able, depending on the presence of an appropriate applied electric field, either to scatter incident light or to transmit the light relatively undisturbed. The fact that such scattering liquid crystal cells can not be switched to a dark opaque state has in general acted as an obstacle towards their utilization for such practical applications as liquid crystal display devices.
The liquid crystal shutter apparatus of this invention is based on the realization that, for applications wherein visual information is repeatedly prevented from arriving at an observer's eyes, it is not a disadvantage that such a shutter device does not become dark but rather an advantage, because during the intervals between consecutive presentations of external visual information the observer's eye is illuminated by an appreciable portion of the incident light scattered by the liquid crystal cell itself. The result is that when the liquid crystal cell is switched between states the illumination of the eye fluctuates much less than it would if the liquid crystal cell were to become dark, such as occurs with liquid crystal cells which operate with polarizing filters. Whereas repeated switching between the transparent and opaque, that is, between light and dark, states can have a fatiguing effect upon the eye, this fatigue is much diminished if the switching is done between the transparent and scattering states since the eye does not have to adapt as much to the incident light of the transparent state.
In general there are two principal means by which liquid crystal materials can be switched, under control of an imposed electric field, between the transparent and the scattering states. One way to achieve this effect is by disturbing the order of the liquid crystal molecules with the electric field. This mode of scattering light is called the dynamic scattering mode and it can be achieved with nematic liquid crystals as well as, in an analogous fashion, with cholesteric liquid crystals. This mode is characterized by a scattering ON-state which is obtained during the imposition of an appropriate electric field, and a transparent OFF-state which is reached after removal of the electric field. The ON-OFF (scattering-transparent) transition is caused by the gradual reorientation of the liquid crystal molecules, due to electrode surface forces, and can sometimes be accelerated by applying an appropriate alternating electric field. Three optical liquid crystal devices which employ this dynamic scattering mode of switching and to which the present invention relates are known from West German Patent Publication DE-A No. 2 111 067, French Patent Publication FR-A No. 2 132 598 and from a report by C. Perrot published in Electronique and Microelectronique, No. 228, 15 Nov. 1976, pp. 39-41. All of these devices are characterized by a transparent OFF-state and a scattering ON-state.
The present invention is based on the second manner of switching liquid crystals between the transparent and scattering states and can be realized with so-called cholesteric liquid crystals, which are normally produced by adding a small amount of cholesteric material to a nematic liquid crystal. Instead of disturbing the order of the liquid crystal molecules with an electric field in order to scatter incident light, as in dynamic scattering, the present liquid crystals are able to scatter light in their natural OFF-state. On the other hand, whenever an appropriate and sufficiently large alternating electric field is applied across the electrodes, the natural helical orientation of the liquid crystal molecules becomes unwound and a transparent ON-state results. The transition between the scattering and transparent states with such mixtures is known as the cholesteric-nematic phase change effect.
The present invention is based on the various advantages of employing the cholesteric-nematic phase change effect, as compared to the switching characteristics of the other optical liquid crystal devices which have been mentioned.
One advantage is that the transition of cholesteric liquid crystals from the (scattering) OFF-state to the (transparent) ON-state occurs at a critical value of the electric field and not at a critical threshold voltage. This critical field value can be varied over a wide range, wherein it can be raised by increasing the concentration of cholesteric in the liquid crystal mixture or by decreasing the separation of the electrode plates. Since a higher threshold field implies a faster switching time, very short turn-ON times can be achieved with cholesteric liquid crystals as compared with other liquid crystals.
Another advantage is that the transition from the (transparent) ON-state to the (scattering) OFF-state is not a gradual and uniform transition which is determined by electrode surface forces, as is the case with other liquid crystals. The nematic-cholesteric transition is a direct function rather of the optical activity of the liquid crystal molecules and, due to the fact that cholesteric liquid crystals are among the most optically active substances known, it is possible to obtain much shorter turn-OFF times as compared with other types of liquid crystals.
Summarizing these two advantages, liquid crystal devices whose operation is based on the cholesteric-nematic phase change effect can be constructed to achieve speeds of switching back and forth between the ON and OFF states which are much faster than can be achieved with either dynamic scattering liquid crystal devices or with liquid crystal devices which rely on crossed polarizers.
A third advantage of liquid crystal devices employing the cholesteric-nematic phase change effect with respect to the other liquid crystal devices which are also capable of scattering incident light concerns the relationship between the imposition and removal of the electric field and the corresponding transparent and scattering states of the devices. For all of these devices the switching times accompanying the imposition of an electric field are shorter than the switching times which result from the molecular relaxation processes which commence subsequent to the removal of the electric field. In order to present visual information to an observer at controlled instants in time and for very short periods of time, it is important that this information be made to appear quickly and without distortion. This requirement can therefore be fulfilled more satisfactorily with a device which quickly switches from scattering (OFF) to transparent (ON) with the imposition of an electric field than with a device which operates conversely. Furthermore, whenever visual information is to be removed from an observer's view, the transition time from transparent to total scattering is less critical than for the complementary transition from scattering to transparent, because whenever information is to be removed, the information will become occulted from the observer's eye relatively soon after the scattering of incident light commences and much before completion of the full transition to the maximum degree of scattering.
To summarize, both the ON-OFF and OFF-ON switching characteristics of liquid crystal optical devices which employ the cholesteric-nematic phase change effect are consequently superior to those of other light scattering liquid crystal devices for purposes of tachistoscopic, i.e. very brief, presentation of visual information to an observer.